1. Laboratory studies in rats. To use elastic scattering spectroscopy (ESS) to detect changes in tissue below the most superficial layer in the gastrointestinal tract. The ultimate purpose of this is to detect residual glands in the esophagus that may persist under regenerated squamous epithelium when PDT is used to treat Barrett's esophagus. 2. Laboratory studies in rats. To use elastic scattering spectroscopy (ESS) to assess the effect of PDT on the mucosa of the rat stomach. The purpose is to find out if ESS can determine whether glandular mucosal ablation is partial or complete and when is the best, and earliest, time after PDT to look. 3. Clinical. To refine ESS analysis for detection of dysplasia in Barrett's esophagus. This will involve a more sophisticated analysis of data we already have, gathering new data to improve the algorithms for spectral analysis and assessing these algorithms prospectively. The possibility of using ESS to detect aneuploidy will also be explored. 4. Clinical. To use OP and ESS to manage treatment and to monitor the results of PDT for dysplasia in Barrett's esophagus. This will include OP measurements of photosensitizer levels at the time of PDT to optimize light dosimetry and ESS measurements of tissue response at times from immediately after light delivery until healing is complete.